1. Field of the Invention
This invention relates generally to an electrical distribution device, such as a circuit breaker and is more specifically directed to providing a circuit breaker having a circuit current limiting contact structure with low watts loss and high withstand level.
2. Brief Discussion of the Related Art
Electrical distribution devices are well known in the art. A conventional circuit breaker includes a pair of contacts which allows circuit current to pass from one contact member to another contact member. An object of these devices is to carry nominal rated current at very low watts loss and have momentary circuit current withstand levels, commonly referred to as "popping levels". A withstand level is generally the level of circuit current that may pass through the circuit breaker before a fault condition is realized causing the contacts to open to prevent circuit current from passing through the contacts. When the contacts open, circuit current is prevented from flowing from one contact member to the other and therefore, circuit current is prevented from flowing to a load which is connected to the device. By having these momentary circuit current withstand levels, operation under high inrush loads, common with motors and transformers, is permitted. Accordingly, these devices need to have momentary circuit current withstand levels so that they may be properly used with such high inrush loads to protect the loads and the overall electrical system.
It is also known in the art to provide a circuit breaker which offers a selective system. A conventional selective system uses large devices, normally air circuit breakers, with very high withstand levels in order to achieve selectivity. The disadvantage with this approach is that these devices do not limit the circuit current to reduce stress on the system. Consequently, these conventional selective systems are designed not to the required load on the system itself, but to the maximum available fault circuit current of the source. Therefore, the conventional selective systems concentrate and are designed in view of the source instead of the required loads on the system. As a result, the designs for the conventional systems are both physically larger and very costly.
Thus, there is a perceived need for a circuit breaker which offers the combination of high withstand and circuit current limiting performance in a single device and in addition the single device provides selectivity with upstream devices and backup protection of downstream devices.